As illustrated in FIG. 1(a), cellular communications networks typically include plural base stations (BS) 101 through which mobile connections 102 are established over the air interface with respective mobile stations (MS) 103. Base stations 101 may be connected through landline(s) to a radio network controller 105, which in turn may be connected by landline(s) to a mobile switching center (MSC) 107 or other switching node. MSC 107 is usually connected, e.g., via a gateway, to other telecommunication networks such as the public switched telephone network (PSTN).
Extension terminals (ET) 109 of the RNC are used to form ATM (Asynchronous Transfer Mode) connections over landlines or links 111 to: base stations 101. Each base station 101 may include, for example, a base station controller 113, a slave timing unit 115 to provide synchronization, and an ATM switching unit 117, and each RNC may include a master timing unit 119, a diversity handover unit 121, and an ATM switch 117 as is known in the art.
In the past, communications between a MS and a BS in such networks have been achieved using Frequency Division Multiple Access (FDMA) and/or Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited via band pass filters which pass substantial signal only within the specified frequency band. Because each channel is assigned a different frequency band, FDMA system capacity is limited by the number of available frequencies as well as by limitations imposed by frequency reuse. In TDMA systems not employing frequency hopping, a channel may consist of a time slot in a periodic train of time intervals over; the same frequency band. A signal's energy is confined to a time slot. In FDMA and TDMA systems, it is not desirable to have two potentially interfering signals occupying the same frequency at the same time. In contrast, CDMA is an access technique which uses spread spectrum modulation to allow signals to overlap in both frequency and time.
Each BS defines a “cell” within which communications may be conducted between the BS and a plurality of different MS units (e.g., cell phones, pagers, etc.) located within the cell. Adjacent cells may often overlap with one another. “Uplink” communications are from a MS to a BS; while “downlink” communications are from a BS to a MS. Different types of cells may operate at different frequencies, even in CDMA type systems. Because MS units tend to move from one cell to another on a relatively frequent basis as MS users travel around, handover procedures must be supported between different cells, and thus between different frequencies, so that MS units which move between cells may have continued support within the network.
There are several conventional techniques for determining which new frequency and/or cell should be selected among plural handover candidates. In certain instances, MS units may aid in the determination of the best handover candidate (and associated new base station). Such aiding may involve the MS periodically or on demand making measurements on each of several candidate frequencies to help determine a best handover candidate based upon some criteria (e.g., strongest RSSI, best BER, etc.).
There exists therefore a need for MS units to be able to efficiently monitor frequencies that are close to the uplink transmission frequency (e.g., for purposes of handover). One way is to use a “compressed mode” type of transmission. In compressed mode, either an increase in coding rate or a reduction in spreading factor (SF) may be used to create a space or a transmission gap (TG) in a frame to be transmitted. Coding rate is indicated by the number of redundant bits per each information bit sent, while SF is indicative of the length of a spreading code, as will be appreciated by those skilled in the art. The idle slots defining the TG collectively have a transmission gap length (TGL). Exemplary compressed mode frames are shown in FIGS. 3-5. TGs in radio frame slots are useful in that a unit may use their idle time to e.g., monitor other frequencies, or perform other tasks.
Compressed mode by reducing the SF results in an increase in bit rate of the physical channels, but the information rate remains approximately constant. A physical channel (PhCH) bit rate is doubled when the SF is reduced by a factor of two. SF is reduced by a factor of two relative to what it normally would be in a normal transmission where all slots in the frame are transmitted with information therein at the information rate. However, when the SF is reduced, power must be increased. For example, when the SF is reduced by a factor of two, this results in a need for an increase in power by a factor of two in order to keep the energy constant as illustrated in FIG. 3.
Assuming an example frame with fifteen (15) slots, a simple reduction in SF by a factor of two (2) results in a TGL of 7.5 slots. Such a TGL may often be longer than necessary, and result in a need for higher peak output power than desired in certain applications, e.g., a doubling of power. It would be desirable to keep such power increases to lower levels.
In addition to reducing a spreading factor (SF), compressed mode may also be achieved by rate matching, e.g., increasing the coding rate on, transport channels (TrCHs) by additional puncturing (i.e., deletion of certain redundant bits). This may achieved by reducing the redundancy of bits sent (i.e., bits are punctured). Compressed mode through rate matching generally means that redundancy is decreased, i.e. bits are punctured, but the bit rate of the physical channel is not altered. Power is then increased to compensate for the reduced redundancy. For example, in a normal mode, fifteen (15) slots per frame are transmitted; while in compressed mode transmission there may be only eleven (11) slots transmitted. In the compressed mode, the TGL is four (4) slots. Power is thus increased by a factor of 15/11 to keep the energy constant. Unfortunately, this rate matching technique is most suitable for obtaining short TGs, and may result in poor performance if extensive puncturing is applied to rate ½ encoded TrCHs.
An object of this invention is to provide improved compressed mode systems/methods in CDMA applications. Compressed mode may be achieved by creating spaces/gaps in frame transmissions. Another object of this invention is to enable a mobile station in a CDMA-based network to monitor other frequencies or perform other tasks during spaces/gaps in transmission.
Another object of this invention is to provided an efficient manner in which to utilize compressed mode in an uplink from a mobile station to a base station in a cellular communications network. TGs in uplink compressed mode frames may be achieved by rate matching and/or SF reduction in different embodiments.
In certain embodiments of this invention, an uplink compressed mode is achieved using both SF reduction and rate matching (i.e., a combination of the two). For example, the SF may be reduced by a factor of two to get twice the bit rate on a channel and the redundancy of bits in the information bit stream may be increased via rate matching to obtain the desired TGL. Thus, there is no need to increase power by a factor of two, and variable length TGLs are achievable, e.g., TGLs having lengths of 1-7 time slots.
In certain example embodiments on an uplink data channel (e.g., on a Dedicated Physical Data Channel, or DPDCH), a reduction of the spreading factor alone by a factor of two may create a TGL of seven and one-half (7.5) slots. However, increased bit redundancy may be used to achieve an actual TGL of five (5) slots. Output power need not be increased as much for a TGL of five slots (power increase by a factor of 15/10 or 3/2), as it would have to be for a TGL of seven and one-half slots (power increase by a factor of 15/7.5=2/1).
In other example embodiments, uplink control channel frames may be formed and transmitted (e.g., on a Dedicated Physical Control Channel, or DPCCH) in compressed mode with TGs. Bit redundancy on the control channel (e.g., of format indicator bits such as TFCI bits) bit may be increased in compressed mode in order to maintain satisfactory performance. In one exemplary embodiment, TFCI bit(s) immediately or directly following the TG may be repeated later in another slot of the frame since these bits may sometimes suffer from slightly worse power control relative to other TFCI bits in the frame. In such embodiments, TGs may be formed using rate matching and/or SF reduction in different embodiments.